Field of the Invention
The present invention concerns devices for detecting and/or measuring visual deficiencies, and more particularly to a device for measuring the level of visual acuity of a patient.
With a view to detecting visual deficiencies and determining the visual acuity of a patient, tests are usually carried out that consist of presenting the patient with real or virtual images of signs or patterns, the dimensions of which are well defined, especially in size and in thickness. These patterns represent, among others, letters or figures, geometrical signs or figures that have been especially devised for this use, such as the Landolt ring, i.e. rings or Landoldt C's, that are open according to a strictly defined geometry.
These images can be panels; they can be projected on a screen or be observed in an optical device under the form of virtual images. They can be produced, for example, from transparencies or slides or other supports that are generally transparent in order to favor the lighting thereof.
Numerous type of devices exist but up to now it has never been possible to overcome certain problems due to the element of doubt that remains for the experimenter with regard to the patient's reactions.
It is known that measuring the visual acuity is appreciated by values of which the maximum is 10, the value 10/10 corresponding to normal visual acuity and the decreasing values to an acuity that is progressively lower.
Various dispositions according to the prior art propose signs corresponding to acuities ranging by increments of tenths from 1/10 to 10/10, the smallest and finest signs corresponding to the highest acuity, the larger and thicker signs corresponding to lower levels of acuity.
This graduation of the sign series shown to the patient will require him to define up to which degree he is able correctly to identify the signs, starting, of course, by the largest ones. The definition of the limit degree is often delicate and the experimenter has, according to the techniques of the prior art, only few means of checking the accuracy of the definition.